1. Field of the Invention
This invention relates generally to a method for producing relatively pure 5-t-butylisophthalic acid by the liquid-phase oxidation of 5-t-butyl-m-xylene in a solvent.
2. Discussion of the Prior Art
Over approximately the past three decades, organic compounds in which two or more carboxylic acid groups are bonded to one or more aromatic nuclei have become of increasing interest, either as direct components in or as intermediates for synthetic condensation polymer molecules. More particularly, one such aromatic polycarboxylic acid, 5-t-butylisophthalic acid, has been employed in the preparation of many polymers, such as polyesters, polyarylates, polyamides and polyarylamides.
Obviously, the presence of impurities in 5-t-butylisophthalic acid can have a serious adverse effect on the physical or chemical properties or performance characteristics of any formulation containing 5-t-butylisophthalic acid itself or any polymer formed from 5-t-butylisophthalic acid. In addition, impurities in 5-t-butylisophthalic acid can adversely affect polymerization processes to which the 5-t-butylisophthalic acid is subjected. Such impurities in 5-t-butylisophthalic acid formed by the liquid-phase oxidation of 5-t-butyl-m-xylene in the presence of a catalyst comprising cobalt, manganese and bromine are often organic impurities or by-products formed during the oxidation and inorganic impurities corresponding to metal components of the catalysts employed in the oxidation or formed therefrom. Such impurities often impart undesirable color characteristics to the 5-t-butylisophthalic acid and its polymerization products.
The major impurity formed in the liquid-phase oxidation of 5-t-butyl-m-xylene to form 5-t-butylisophthalic acid in a C.sub.2 -C.sub.6 monocarboxylic acid solvent and in the presence of an oxidation catalyst comprising cobalt, manganese, and bromine components is trimesic acid, which is formed by the oxidation of the t-butyl group in 5-t-butyl-m-xylene. Trimesic acid is a trifunctional acid and will cause branching in polymers, which at high levels leads to brittleness. In addition, the formation of trimesic acid represents a reduction in the yield of 5-t-butylisophthalic acid.
Thus, maximization of the yield of 5-t-butylisophthalic acid and minimization of the formation of undesirable by-products, especially trimesic acid, during the oxidation of 5-t-butyl-m-xylene, and the removal of such by-products from the resulting crude 5-t-butylisophthalic acid product are highly desirable. One of the difficulties with the prior art procedures for the aforesaid oxidation and purification lies in their inability to provide an essentially pure product without expensive and time-consuming separation procedures. Generally, where polyalkyl derivatives are employed as reactants, the reaction sometimes appears to proceed with the oxidation of one of the alkyl radicals to the exclusion of the remainder. Such by-products frequently tend to possess solubility properties similar to those possessed by the desired product, making separation and purification of the desired product expensive and time-consuming in commercial operations. A further difficulty encountered in such reactions involving the formation of intermediate oxidation products resides in the fact that conditions and catalyst materials which function most efficiently at one stage of the reaction may become less efficient for oxidation at another intermediate stage.
Furthermore, the removal of organic and inorganic impurities from aromatic polycarboxylic acids, including 5-t-butylisophthalic acid, formed by the catalyzed, liquid-phase oxidation of polyalkyl aromatics is typically very difficult, and the removal technique employed depends on the specific aromatic polycarboxylic acid from which the impurities are to be removed and the specific oxidation conditions and catalyst employed to make it. Furthermore, techniques for purifying aromatic polycarboxylic acids are often relatively time-consuming and involve relatively complex reaction schemes.
For example, Hensley et al., U.S. Pat. No. 3,344,177 illustrates the complexity of prior art purification methods. This patent discloses in Example 11 a method for purifying crude t-butylisophthalic acid that was prepared by the liquid-phase oxidation of t-butyl-m-xylene in acetic acid with air and in the presence of a bromine-promoted heavy metal oxidation catalyst, and then by washing the solid t-butylisophthalic acid with acetic acid and drying the washed t-butylisophthalic acid. Hensley et al. does not disclose the atomic ratio of bromine-to-heavy metal catalyst component or the temperature employed for the oxidation. The crude t-butylisophthalic acid is dissolved with aqueous sodium hydroxide. The resulting solution is filtered and acidified to a pH of about 6.0. A portion of the solution containing 3 pounds of dissolved crude acid is percolated at ambient temperature through 2.5 pounds of PCC CAL activated carbon in a column two inches in diameter and four feet long. The percolation effluent is filtered to remove carbon fines (no pretreatment of carbon bed with water), diluted 1 to 1 with water, and admixed with a 20% sulfuric acid solution heated to 194.degree. F. to a pH of about 2 to 3 to regenerate solid t-butylisophthalic acid. The precipitated t-butylisophthalic acid is recovered by filtration; the filter cake is washed with distilled water, slurried in boiling distilled water, and recovered again by filtration and dried at 140.degree. F.
Therefore, it is highly desirable to produce 5-t-butylisophthalic acid under conditions such that the production of impurities and their incorporation in the crude 5-t-butylisophthalic acid product are minimized and the yield of such higher quality 5-t-butylisophthalic acid is improved and to purify the crude 5-t-butylisophthalic acid by a relatively simpler and shorter procedure to obtain 5-t-butylisophthalic acid having the requisite purity.